Five-week-old male mice were fed: (I) 0.3, 1, 3, 10, 30, 100 and 300 ppm of 3,4,5,3',4',5'-hexachlorobiphenyl (HCB); (II) 10, 30, 100 and 300 ppm of 2,4,5,2',4',5',-HCB; (III) 10, 30, 100 and 300 ppm of 2,3,6,2',3',6'-HCB and (IV) 10, 30, 100 and 300 ppm of 2,4,6,2',4',6',-HCB. Surviving mice were sacrificed at 28 days. HCB levels in adipose tissue and liver were determined. There were variations among the isomers as to dose and pathologic effects. Isomer (I) showed the greatest effect of those studied on mortality, body weight gain, liver, thymus, spleen, and testes; it also attained the highest tissue concentration. It was the only isomer which produced porphyrin accumulation. The decreasing order of overall toxicity was I greater than IV greater than II greater than III.